Power supply fob electron



April 22, 1947. A. w. VANCE 2,419,428

POWER SUPPLY FOR ELECTRON MICROSCOPES Original Filed Nov. 15, 1940 3Sheets-Sheet 1 1960 V. P5611419 7E0 Bil/M750 3nventor April 22, 1947. w,VANCE 2,419,428

POWER SUPPLY FOR ELECTRON MICROSCOPES Original Filed NOV. 15, 1940 3Sheets-Sheet 3 4 r AAlll jllll I w COIL 3nvcntor Patented Apr. 22, 1947UNITED STATES PATENT OFFICE POWER SUPPLY FOR ELECTRON MICROSCOPES ArthurW. Vance, Cranbury, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application April 28, 1942, Serial No. 440,771,which is a division of application Serial No. 365,750, November 15,1940, now Patent No. 2,302,900, dated November 24, 1942. Divided andthis application April 17, 1943, Serial No.

- v 2 Claims. (Cl. 171-312) 1 2 The subject invention relates to powersupply with respect to ground. It is a further object of systems forelectronic devices and has for its this invention to eliminate thenecessity of supprincipal object the provision of an improved plyingdirect current for the electron gun by method and means for producing ahighly stabienergizing the filament with radio frequency curllzed sourceof voltage for use with the electron 5 rents. A still further object ofthis invention is microscope whereby greatly improved performto providean improved high voltage rectifier cirance may be obtained. cuit havinga feedback stabilizer which permits This application is a division ofapplicants cothe required high voltage to be obtained with less pendingU. S. application, Serial Number 440,771, equipment and fewer parts thanheretofore been filed April 28, 1942, which is a division of U. S.required, thus greatly reducing the cost and ize application, SerialNumber 365,750, filed Novemof the power supply system. Still furtherobjects ber 15, 1940, upon which U. 5. Patent 2,302,900 of the presentinvention are to provide an imwas granted to applicant November 24,1942. proved automatic relay and protective circuit; to

The development of the electron microscope provide an l pr e Os o for ppy the has presented new problems in the design of high ra io q e cyurren i ized in e i in the voltage power supplies and, in the design ofcurhigh Voltage ifier; and to p o d p oved rent regulating systems forthe various electror e t ulators for use in conju c With t e magneticcoils which constitute the magnetic 1ens microscope len-se i system ofthe microscope. In order to produce Briefly, these objects areaccomplished by ema beam of high velocity electrons, an electron gunploying a source of radio frequency currents is utilized which isenergized with a direct curcoupled to a, voltage doubling rectifiercircuit in rent voltage which may be as high as 100 kilowhich thfilament of one of the rectifier tubes is volts. The current load of thehigh voltage source energized with radio frequency currents, thus is ofthe order of 1 milliampere, and the problem, facilitating filteringproblems, deriving a potentherefore, is one of voltage stability. Themag tial corresponding in amplitude to the rectified netic lensesrequire currents of the order of 400 output, and utilizing thispotential to control the milliamperes which likewise must be maintainedoutput of the oscillator supplying the high freconstant over shortperiods. quency currents to the rectifying system.

The stability of the voltage supply system for This invention will bebetter understood from an electron microscope is measured over the 39the following description when considered in conperiod of time requiredto properly expose a phomotion with the accompanying drawings in whichtographic plate used to record the images. In Figure 1 is a blockdiagram of the essential elepractice, the longest exposure isapproximately ments of a complete power supply system for an seconds.Consequently, the voltage supply syselectron microscope; Figure 2 is anequivalent tem must not vary appreciably within this period, circuitdiag illustrating the Operation of the since slight variations in theelectron. speed or in high voltage rectifier and filter system; Figure 3the focus of the magnetic fields will defocus the is a simplifiedcircuit diagram of the Sy electron image and distort the photograph.cluding the high voltage rectifier; Figure l is the It is also necessaryto consider the short-time circuit diagram of the driver oscillatorwhich constancy of the power supply system. since this supplies highvoltage radio frequ y currents to limit the resolution of themicroscope. Theothe high volt rectifier; Figure 5 is h ir uit reticalcalculations based on a resolving limit of diagram of :a D.-C.modulating amplifier; Figure 10 Angstrom units indicate that the mostcritical 6 is a circuit diagram ot a protective circuit incurrent mustnot vary more than .OOt of one pereluding an overload relay; Figure 7 isthe circuit cent and that the high voltage should not vary diagram of areulat d urr t s p y suitable for more than .096 of one percent. use inconjunction with the electromagnets of the It is well known that theelectron beam must lens system; and Figure 8 is an alternative ou bethoroughly shielded from all external fields. rent regulating system.

The difficulty of shielding magnetic fields produced by 60 cyclecurrents has heretofore necessitated the use of direct current to supplypower Referring to Fig. 1, a complete power supply to the filament ofthe high velocity electron gun. system for an electron microscope isillustrated Since the cathode is normally operated at a high in blockdiagram form. Power is derived from a negativ potential, with the anodegrounded, the 60 cycle power line 9 and applied to a number filamentsupply is necessarily at a high voltage of regulated power units whichenergize various !The power supply system components of the system andwhich are designed to have output voltages which are determined by theirintended use. The regulated power supply I I energizes a,pair of lowfrequency oscillators l3 and i5 which are utilized, respectively, toenergize the filament of the electron microscope and the filament of oneof the high voltage rectifiers. The high frequency-high voltagerectifier and filter circuits are represented by the block (I and willbe described in detail hereinafter. Another power unit (9, which may beunregulated, energizes a driver oscillator 2| which generates highvoltage radio frequency currents for the high voltage rectifier. Amodulating amplifier 23 is energized through a connection 22 by theunregulated power unit I9 and is also supplied with a voltage over alead which is proportional to the rectified output voltage. This voltageis compared to that supplied to the modulated amplifier by a standardbattery 21, in a circuit which will be described in detail hereinafter,to produce an output control voltage which is utilized to control theamplitude of oscillation of the driver oscillator 2|. This controlconnection is illustrated by lead 29. The amplifier is also connected bya lead 20 to a regulated power source 3! which supplies a negativevoltage of the order of 350 volts, the purpose of which is to bedescribed later.

Another regulated power unit 33 supplies a direct voltage of the orderof +400 volts to the driver oscillator 2| and to the modulatingamplifier 23 through a lead 35. In order to achieve the highest degreeof accuracy, the modulating amplifier 23 and, in addition. a currentregulator 3! for the objective coil are supplied with a regulatedcurrent through connection 24 from a source 39 which is used to energizethe filaments of certain of the tubes of these devices. Regulation inthe current regulator 39 is achieved by utilizing the 350 volts obtainedfrom the regulated power supply by a. connection 32 as a standardreference voltage, in a manner which will be described hereinafter. Theprojection coil and the condenser coil 4! and 43 are energized bycurrent regulators 45 and 41, which derive power from power supplies 42,44. respectively, both regulators having a connection over conductor 32to the regulated power supply 3!. A particular advanta e of this systemof interconnect on i that one power supply which is carefully regulatedcontrols the output voltages of the other power supply units, andcontrol by a regulated or standard voltage is efiectuated in a much moreeconomical manner than by any other method.

The high voltage rectifier Referring now to Fig. 2, a circuit diagram ofthe high voltage rectifier system is shown. Since an extremely highvoltage is produced, the high voltage equipment has been mounted in anoil tank 5| which not only protects the user from harm, but alsodecreases the danger of accidental fiashover or short circuits. Thedriver oscillator M is connected to a series resonant circuit comprisingan inductance Li) and a capacitance C0, the latter representing theeffective lumped capacity between the point 54 and ground due to theprimary of transformer T2, and the rectifier tubes VI and V2 in serieswith the capacitor CI. The cathode of the first rectifier VI isgrounded, while its anode is connected to the inductor L0 through thecoupling capacitor Cl and also the cathode of the second rectifier V2.The anode of the second rectifier is connected to the oathode of thehigh velocity electron gun of the microscope and is the source of highvoltage direct potential whose polarity is negative with respect toground.

The filament of the first voltage rectifier VI is supplied from aconventional 60 cycle line, while the filament of the high voltagerectifier V2 is obtained from the filament oscillator H! which isconnected to input terminals 53, the circuit including a couplingtransformer T2 having an untuned secondary and a tuned primary.

Radio frequency actuation of the high voltage rectifier has a number ofadvantages over the use of a conventional 60 cycle alternating powersource for the production of high voltages of great constancy at lowcurrent drains. For example, much smaller filter condensers are requiredfor a given ripple output. This means that the stored energy in thefilter circuit is very much less than that of the usual 60 cycle "bruteforce filter system. The importance of thi is that in case of aflash-over in the high voltage circuit, due to a loss of vacuum in themicroscope, for example, the stored energy is not sufilcient to burn upthe equipment, as was the case in the earlier microscopes. Sinceresonant circuits are used, the ripple fed through the rectifiercapacitor is sinusoidal and consequently can be resonated out to a largedegree. Furthermore, by using low loss coils in the resonant circuit, anextremely high impedance may be realized which occupies but a smallspace and is extremely light in weight as compared to an equivalentimpedance at the conventional power frequency. Also the exciting powerrequired by such a coil is greatly reduced.

The use of radio frequency high voltage power supply also improves theoperation of the output control system. When the control is operated onthe low voltage input side of the rectifier, as is desirable, the speedof control is limited by the frequency of the supply. Consequently, thislimitation is negligible where the supply voltage is a radio frequencyvoltage.

The use of radio frequency to energize the cathode or filament of thehigh Velocity electron gun has two important advantages. In the firstplace, the microscope may be readily shielded from the stray highfrequency fields which are produced, and this result is aided by thefact that no 60 cycle high voltage transformers are required which havevery large external fields. In the second place, the use of radiofrequency eliminates the necessity for employing bulky storage batteriesor their equivalent.

The operating frequency for the rectifier system is not critical. Theproblem is essentially that of obtaining the required output voltagewith a minimum exciting power. This, of course. requires obtaining themaximum resonant impedance of the high voltage coil. The resonantimpedance is given by the formula:

wL RL+RC 5 and R1. and Re are the effective series A.-C. resistances ofL and C, respectively. In the frequency region from approximately 20 kc.up to several hundred kc, the maximum Q possible in a coil of givenvolume is more or less independent of frequency; thus, the coil should,in general, operate below 50 kc.

The rectifier circuit is seen to be of the type wherein the inversevoltage of the half wave rectifier VI is rectified by V2, therebycharging the output capacitor C2 to a voltage nearly equal to twice thepeak voltage which appears across the inductance L0. It is to benotedthat one side of the input circuit is grounded and that no primarywinding is required on the high voltage coil.

The filament transformer T2 has a low interwinding capacity which ispreferably of low power factor. The primary and secondary windings mustbe spaced sufilciently to withstand the peak voltage output, and, as aresult, a large amount of energy must be stored in the tuned p y inorder that the secondary may absorb the power necessary to excite thefilament. Preferably, the tuned primary of transformer T2 constitutesthe tank circuit of the driving fila. ment oscillator. The size of thesecondary is selected to match the impedance of "the filament load andto provide the desired voltage. The filament current may be controlledconveniently by varying the frequency of the oscillator {5.

Fig. 3 shows the rectifier circuits connected to the other elements ofthe power supply system. It will be noted that the negative voltage forthe electron gun cathode obtained from the output of rectifier V2 isapplied to a center tapped resistor 57 connected across the secondary ofthe transformer T3 which supplies radio frequency current to thefilament of the electron gun 59. The filament, as a whole, is 60kilovolts below ground potential and must therefore be carefullyinsulated. The radio frequency current supplied by the filamentoscillator I3 is applied to the filament through a coaxial cable 6 I.

It will be noted that the filter capacitor C2, .005 microfarads, forexample, is connected to ground through a parallel connected coil L5 andcondenser C6. The values of these reactors are such that at theoperating radio frequency, ef fective inductive reactance of L5 and C6res onates with capacitor C2 and forms an effective ground for ripplevoltages appearing in the output circuit. At a higher frequency, forinstance, midway between the fundamental and second harmonic L5 and C6become parallel resonant, presenting a high impedance, but as there isno ripple at this frequency anyway, no harm results. At higherfrequencies including the sec ond harmonic, the shunt capacitor C6 inseries with the output capacitor C2 provides an efiective ground. Aspark gap 63 is connected across the shunt reactors L5 and C5 in orderto discharge voltage surges without causing a breakdown of the elements.

High voltage regulator oscillator 2|, a voltage divider comprising ahigh resistance capacity-compensated high voltage resistor 65, 1100 meg,and a relatively low resistance resistor 61, 10 meg, serially connected.

between the high voltage source and the regulator. The lower end ofresistor 61 is bypassed to ground and connected through an isolatingresistor to an adjustabletap on a standard battery 69, the negativeterminal of which is ground ed. The standard voltage produced by thebattery 59 is therefore compared to the divided voltage appearing acrossresistor 67, so that no input voltage is applied to the amplifier 23when the divided voltage obtained from the high voltage output isexactly equal to that of the standard battery. By varying the taps onthe standard battery 69, the output voltage may be controlled, forexample, in 5 kv. steps from 30 to 60 kv.

The details of the driver oscillator which supplies high voltage for therectifier tubes VI and V2 is shown in Fig, 4. Since the resonantfrequency of the series resonant inductor L0 and C0 included within theoil tank 5| varies considerably with temperature, a masteroscillatorpower amplifier is impractical without automatic frequencycontrol to keep it at resonance. It is therefore proposed to utilize aself-oscillating circuit whose frequency is determined wholly by theresonant frequency of the load L0, C0. This oscillator must also becapable of modulation over a considerable range in order to providecontrol over the output voltage. The circuit utilized comprises atwo-stage oscillator including tubes V3 and V4, the output of the latterbeing coupled to the input of the former by a conventional impedancecoupling system, tube V4 being shunt fed through inductor L4 and coupledto the resonant circuit load through a small series resistance R.Inductor L4 is resonated at the operating frequency by a capacitor C5. Afeedback voltage is obtained by the drop across resistor R, theterminals of which are connected to the primary of a shieldedtransformer H, the secondary of which is coupled to the input of thefirst oscillator tube V3. The transformer H preferably has a broadfrequency response, this being accomplished, for example. by a dampingresistor and capacitor 13 and T5. The feedback gain is sufficient onlyto sustain oscillation at or very near the resonant frequency of theload Lo, 00.

The amplitude of oscillation is controlled by varying the screen gridpotential of the output tube V4. This is illustrated in Fig. 4 by thepotentiometer connection shown in dotted lines, the potentiometerrepresenting the control bias derived from the D.-C. modulatingamplifier 23. The details of this connection are illustrated in Fig. 5.The tank circuit L4, C5 has a high L/C ratio and it therefore exertsonly slight control over the frequency of oscillation. Its principalfunction is to maintain reasonably sinusoidal voltage conditions in theplate of V4.

Referring to Fig. 5, a unique D.-C. amplifier is illustrated suitablefor use in applying the small variable D.-C. voltage of the controlsystem to the driver oscillator to effectuate control of the amplitudeof oscillation. Resistor 6! corresponds to the similarly numberedresistor in the voltage divider circuit of Fig. 3. At any instant, thepotential of the lead 22 connecting this resistor to the control grid ofthe first amplifier tube 1'! is equal to the sumof the negative dividedvoltage and the voltage due to the standard battery 99. The latterbattery is preferably tapped in steps and constitutes the. main controlof the high potential output.

The filament of the first amplifier tube T! is connected to a regulatedcurrent source, which may be of the type illustrated in Fig. 8 and 7 Lhereinafter described. in orde r to assure constant electron emission.Screen grid potential is obtained from a voltage divider 19-8I connectedto a suitable source of positive potential which is applied to terminal83. Plate voltage is obtained from a resistor I connected between theplate and the terminal 83. The plate is connected to the control grid ofthe second amplifier tube 85 through a parallel connected resistor 81and capacitor 89. A phase control network comprising resistor SI andcapacitor 93 is connected in shunt with the plate of the amplifier tube11. Screen grid potential for the second amplifier tube 85 is derivedthrough a dropping resistor 95 from a suitable source of positivepotential which is connected to a terminal 91. A gas filled regulatortube 99 is utilized to control the screen grid potential. The anode ofthe output tube 85 is connected through an anode load resistor IOI tothe positive supply terminal 83 and also through an isolating resistorI03 to the screen grid of the second oscillator tube V4, which is shownin Fig. 4.

It will be noted that the plate voltage of the first amplifier tube 11is impressed on the grid of the second amplifier tube 85 through thecoupling resistor 81. Since the cathodes of the two amplifier tubes areboth operated at ground potential, it is necessary to overcome theefi'ect of the plate voltage on the grid of the second amplifier. T hisis accomplished by connecting a source of negative voltage, for example,350 volts derived from the voltage supply 3| illustrated in Fig. 1,through an isolating resistor I01 to the control grid of the outputtube. This has an effect similar to that of the usual series buckingbattery in D.-C. amplifiers, but has the advantage that one terminal ofthe source of negative voltage is operated at ground potential, which isnot true in the conventional case.

It has also been found that it may be desirable to drive the screen gridof the oscillator tube more negative than is possible by direct controlfrom the amplifier tube 85. This is accomplished by connecting the -350volt source tothe lead I09 through a resistor III, thus reducing theaverage potential of the screen and making it possible to drive thescreen more negative when the output tube 85 is drawing maximum platecurrent.

Protective circuit It has been found that, due to the high voltage used,there is some danger of arcing at the high voltage terminals. This mayoccur within the vacuum chamber of the microscope, due to a failure ofthe vacuum, for example, or it may con-r by reason of a failure of theinsulation of the high voltage condensers.

Referring to Fig. 3, it will be noted that the ground return of the highvoltage supply flows through the output current meter A. In order toprotect the meter from danger, a protective circuit 62 has been providedwhich absorbs sudden current surges and limits the current tosubstantially the normal value. In order to protect the apparatus fromsustained overloads, a cutout or through resistors H1, H9, I2I, meter A,resistor I23, the energizing coil of an overload relay I25 and aresistor I21. A neon regulator tube I29 and a pair or" condensers I3Iand I33 are connected between the output terminals of the meter A andthe junction points of the resistors III, II 9 and I 2|, respectively."The output terminal of the meter A is also connected to ground througha limiting neon tube I35.

The overload relay circuit includes a switch I39 which, in its normalposition, makes contact to a grounded terminal I. When actuated by anoverload current, the relay armature makes contact to a terminal I43which is connected to a source of negative voltage, for example, the-350 volts provided by the power supply II. The armature is connected tothe grid return of the oscillator tubes of the driver oscillator asIllustrated in detail in Fig. 4. The armature is also connected througha push button I41 to the relay coil through a current limiting resistorI49. The same armature is also connected to a voltage divider I5I acrossa portion of which a neon indicator tube I53 is connected.

A current surge, caused by an arc or breakdown of a high voltageterminal, flows through the limiting resistors III, H9 and I2I andcharges capacitors I3I and I33. In addition, the voltage across theoutput meter A is limited by the neon tube I29. The meter is thereforeprotected from damage. The sustained overload fiows through theactuating coil of the relay I25 and connects the armature to thenegative potential source, thus removing the normal ground connectionand applying a high negative potential to the driver oscillatorsufilcient to stop its oscillation. Since the driver oscillator providesthe high voltage for the high voltage rectifier system, it will beapparent that the relay immediately shuts oil the high voltage supply.At the same time, the relay connects a. holding circuit through the pushbutton I41 so that current flows through a resistor I55 and through therelay to hold it in its closed position. It will remain in this positionuntil the circuit is broken by operating the push button I41. If thefault has cleared, the overload relay will open, restoring the highvoltage to the rectifiers. If the fault has not cleared, the highvoltage will not remain on. indicating that the circuit must be checked.When the overload relay is actuated, the neon output indicator I53 willlight, thus providing a visual indication to the operator that the highvoltage has been cut 0115.

Low voltage or current regulators A current regulator of the typepreferred for use with the projection and objective coils of themicroscope is illustrated in Fig. '1. A conventional rectifier I51 andfilter I59 supply a high voltage to the plate of a triode I BI, thecathode 01' which is serially connected to ground throughthe projectionor objective coil I63 and current control resistors I65 and I51, theformer being adjustable. The voltage drop across the latter resistors iscompared to the voltage of a. standard battery I59 and applied, througha relatively high resistance I10, to the grid of a. regulating tube III,the plate of which is energized by a suitable source of positivepotential connected to an input terminal I13 through a plate resistorI15. The plate 01' this control tube is connected to the grid of thetriode I5I through a coupling resistor I 11. A by-pass capacitor I13 isconnected between the grid of the regulating tube III and the 2,41a4ascathode of the triode lGl. The high resistance I70 and the by-passcapacitor l'l8 thereby comprise a filter for eliminating from theregulator output circuit substantially all objectionable hum and allexcept extremely low frequency noise components. The filter I78, I78 ineffect provides voltage regulation of A.-C components in the circuit.

It will be observed that the cathode of the triode l6l is positive withrespect to ground. However, the plate of tube l'll cannot becomesufficiently less positive than the cathode of tube llil to provide thenecessary bias for the latter tube under conditions of low outputcurrent. In order to supply the proper voltage to the grid of the tubel6l, it is connected through an isolating resistor I19 to the source ofregulated negative voltage, as in the case of the D.-C. amplifierdescribed above.

A current regulator of somewhat simpler form which does not require astandard battery is illustrated in Fig. 8. This amplifier is preferablyused for the condenser coil of the microscope and may also be used toregulate the filament current of the D.-C. amplifier and the controltube I'll of the current regulator illustrated above in Fig. 7. The unitmay also be considered as a voltage regulator since the voltage across aconstant impedance is constant when the current through it is constant.Consequently, the same control circuit is employed, for example, inregulating the output of the regulated 800 volt power supply ll.

As before, the conventional rectifier and filter l5! and I59 areconnected to the plate of a limit ing triode ii, the cathode of which isconnected to ground through the electron microscope condenser coils orthe regulated filament, as the case maybe, and through the control andadjusting resistors I85 and lei, The voltage drop across the latterresistors is compared to a voltage derived from the regulated 350 voltsobtained from the power supply 3l, thus eliminating the standardbatteries I69 utilized in the preceding regulator. A particularadvantage of this method is that the standard voltage source isoperatedwith one terminal grounded, which is not possible where standardbatteries are used in a series circult. Since the cathode of the triode-l6l is positive with respect to ground, a noted above, it may not benecessary to apply the auxiliary negative potential to the grid of thistube so that in this case theplate of the control tube l'll is connecteddirectly to the grid of the triode l6l. It will be apparent that in bothcases, changes in the current through the coil and control resistorsproduces a variable voltage drop across the resistors which is appliedto the control tube l'll to vary the amplitude of the current in adirection tending to compensate for the change,

The measured stability of the power supply system herein described hasbeen found to be well in excess of that required for stability anddefinition 01 an electron microscope, Several hundred photomicrographshave been successfully taken with no indication that the results havebeen limited by variations of the power supply. This is in distinctcontrast with' microscopes of the prior art in which a large percentageof the photomicrographs is spoiled by reason of changes, in the powersupply voltage. The system is so stable that the microscope may be resetto previous conditions without observing the electron image or thefocusing, Exposures thus made have good 10' resolution. In addition, itis possible to make wide variations of the image intensity withoutchanging the focus.

A particularly severe test which was successfully passed by the electronmicroscope operated in conjunction with the power supply and controlsystem of the present invention is that of reducing the vacuum duringoperation until an internal arc takes place, causing the overload relayto actuate, then restoring the vacuum, and applying the high voltage toobtain the original picture exactly in focus without readjustment.Previous- 1y known microscopes could not be treated in this manner andgenerally had to be taken apart and repaired after an internal flashoverdue to the severity of the discharge from the filter.

I claim as my invention:

1. A current regulator comprising a source of direct current, a loaddevice, an electron discharge device having cathode, anode and gridelectrodes, said anode and cathode electrodes being connected in seriescircuit with said load device and said source, said series connectionalso including a series impedance, a control tube having cathode, gridand anode electrodes, means coupling the grid and cathode of saidcontrol tube in shunt with all of said impedance, said grid of saidcontrol tube being connectedto the positive terminal of said impedance,means conductively coupling the anode of said control tube to the gridof said discharge device, means for applying a positive potential tosaid anode electrode of said control tube, and a network connected tosaid grid of said control tube for suppressing A.-C. potentials acrosssaid impedance.

2. A current regulator comprising a source of direct current, a loaddevice, an electron discharge device having cathode, anode and gridelectrodes, said anode and cathode electrodes being connected in seriescircuit With said load device and said source, said series connectionalso including a series impedance, a control tube hav ing cathode, gridand anode electrodes, means coupling the grid and cathode of saidcontrol tube in shunt with all of said impedance, said grid of saidcontrol tube being connected to the positive terminal of said impedance,means conductively coupling the anode of said control tube to the gridof said discharge device, means for applying a positive potential tosaid anode electrode of said control tube, separate means for applying anegative potential to the grid of said discharge device to compensatefor the positive voltage applied thereto through said control tube anodecoupling means, and a network connected to said grid of said controltube for suppressing A.-C, potentials across said impedance.

ARTHUR W. VANCE.

REFERENCES CITED.

The following references are of record in'the file of this patent:

UNITED STATES PATENTS Number Name Date 1,858,271 Harness May 17, 19322,075,966 Vance Apr, 6, 1937 2,117,138 Bock May 10, 1938 2,210,394Braden Aug, 6, 1940 FOREIGN PATENTS Number Country Date 472,326 BritishSept, 22, 1937

